THE NATIONAL GEOGRAPHIC MAGAZINE
Photograph courtesy Bureau of Biological Survey
A BEAVER SKULL SHOWING THE CHISEL-LIKE INCISORS USED IN CUTTING TREES
The large incisors are operated by powerful muscles attached to the massive bones of the
skull, and the chisel edges are kept sharp by frequent scraping away of the soft inner dentine
from the thin surface layer of hard enamel. Often when a beaver seems idle it can be heard
sharpening its teeth by rubbing the edges together. If one curved incisor is injured in the jaw
it may grow no more, but its complement sometimes extends and finally seals the jaw, causing
death from starvation.
time for Mickey's regular appearance, the
water of the pond would have dropped
about 12 inches in level. Ninety minutes
of strenuous work by Mickey would result
in stoppage of the flow by a superficial
upstream face (see pages 742 and 747).
Next morning repair was complete and
one could not recognize the place of em
brasure except by its relation to other ob
jects.
Repeatedly Mickey's wife appeared,
coming near, as if she intended to help in
the repair work. Two young were with
her on each visit. Always her advent re
sulted in Mickey's slap of alarm and her
disappearance with the children.
Mickey's initial conduct in the daily re
pair of the break, after a general inspec
tion, which took him over the wall in a
careful examination of the downstream
base, was to fell several saplings. He
towed them to the breach and allowed
them to float down, the butts resting on
the creek bottom below. The angle at
which he placed them left about six feet
of top with branches protruding above the
wall.
These saplings were from 2 to 5 inches
in diameter. Often he utilized inundated
stock. His dive would indicate the tree
he was going to cut in the water. Several
times he cut stock of more than two inches
without coming to the surface during the
operation. His submerged time on a tree
of this size was about II minutes. At in
tervals bubbles would arise, denoting an
extra supply of air when he dived.
With his "studding" of saplings in place,
nondescript bits of flotsam were assem
bled in a depression of the wall. On one
occasion, not until he had brought in seven
small stakes, which he drove down verti
cally at intervals in the breach, did he
make use of this cache. To me this was
obvious planning in his utilization of
building material.
He collected willows of about an inch in
width, and drove them into the sides of
750